Early voltage in mosfets
This presentation is the property of its rightful owner.
Sponsored Links
1 / 22

Early Voltage in MOSFETs PowerPoint PPT Presentation


  • 80 Views
  • Uploaded on
  • Presentation posted in: General

Early Voltage in MOSFETs. Due to channel length modulation:. Saturation Voltage. V pinchoff = V DS,sat = V GS – V TH Separates resistive from saturation region The drain current is given by Solving for V DS,sat : Good to solve for quiescent voltage-current.

Download Presentation

Early Voltage in MOSFETs

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Early voltage in mosfets

Early Voltage in MOSFETs

  • Due to channel length modulation:

Brad Noble

Chapter 3 Slides


Saturation voltage

Saturation Voltage

  • Vpinchoff = VDS,sat = VGS – VTH

    • Separates resistive from saturation region

  • The drain current is given by

  • Solving for VDS,sat:

  • Good to solve for quiescent voltage-current.

Brad Noble

Chapter 3 Slides


Ex find v ds sat for an nfet

Ex: Find VDS,sat for an NFET

Brad Noble

Chapter 3 Slides


Variations in v th across channel

Cox

Gate oxidecapacitance

inversionlayer

Depletion cap,function of x

Cdep

Variations in VTHAcross Channel

  • We assume VTH is constant across channel

    THIS IS NOT TRUE!

  • Depletion region is thick at S and thin at D.

Brad Noble

Chapter 3 Slides


Notes on pfets

Notes on PFETs

  • PFETs typically have a shape factor 3 or 4 times larger than NFETs

  • Body effect can be eliminated in PFETs by tying the n-well to VDD

    • Need 6m spacing between n-wells to isolate.

    • Dr. Engel always does this on input devices, not always elsewhere.

Brad Noble

Chapter 3 Slides


Weak inversion

Weak Inversion

  • What really happens if VGS < VTN?

  • In digital design, IDS = 0.

  • We call it “weak inversion” or W.I.

  • IDS is primarily due to Idrift in strong inversion and Idiffusion in weak inversion.

Brad Noble

Chapter 3 Slides


Modes of inversion

Modes of Inversion

  • IDS = Idrift + Idiffusion

  • If VGS > VTN the channel has been inverted.

  • To be more precise, we can say the channel has been “strongly inverted” (S.I.) due to an abundance of carriers in the channel.

  • Inversion is independent of whether the FET is in the linear or saturation region.

Brad Noble

Chapter 3 Slides


Weak inversion i diffusion

Weak Inversion Idiffusion

  • Drain is more reverse biased than source:

  • To find Idiff, compute gradient

  • Because no carriers are lost as they travel from S to D, current is the same for all x and gradient is not a function of x.

  • Note: This is not really true due to recombination, but its close!

Brad Noble

Chapter 3 Slides


W i surface potential

W.I. Surface Potential

Brad Noble

Chapter 3 Slides


Deriving weak inversion i ds

Deriving Weak Inversion IDS

Brad Noble

Chapter 3 Slides


W i fet as exp law dev

W.I. FET As Exp. Law Dev.

  • S must be big for device to be useful.

  • If VDS = 100mV, can be neglected.

  • For W.I. vDS,Sat 100mV

  • Looks like a BJT

Brad Noble

Chapter 3 Slides


Inversion coefficient

Inversion Coefficient

  • Let

  • Shape factor as a function of  :

    Lets you chose shape to match inversion mode.

Brad Noble

Chapter 3 Slides


Ex using inversion coeff

Ex. Using Inversion Coeff.

Brad Noble

Chapter 3 Slides


Small signal analysis

Small Signal Analysis

Brad Noble

Chapter 3 Slides


Ex quiescent point

Ex: Quiescent Point

Question: How many digits are significant?

Brad Noble

Chapter 3 Slides


Small signal model limits

Small Signal Model Limits

  • Suppose the previous circuit is the input device of an amplifier.

  • Small-signal model holds as long as the deviations are small

Brad Noble

Chapter 3 Slides


Taylor series expansion

Taylor Series Expansion

  • Taking a Taylor expansion of one variable:

Brad Noble

Chapter 3 Slides


Small signal model params

Small Signal Model Params

Brad Noble

Chapter 3 Slides


Example small signal analysis

Example: Small Signal Analysis

Brad Noble

Chapter 3 Slides


Small signal low freq model

Small Signal Low-Freq Model

Brad Noble

Chapter 3 Slides


Ex find g m and r o

Ex: Find gm and rO

Brad Noble

Chapter 3 Slides


Transconductance w i m i

Transconductance: W.I. & M.I.

  • What is gm for a weakly inverted FET?

  • What is gm for a moderately inverted FET?

Not in textbooks!

Brad Noble

Chapter 3 Slides


  • Login